Water Vapor Spies
Fifty years ago scientists began measuring carbon dioxide continuously on top of Hawaii's Mauna Loa; this year they'll set up the first real-time experiments there to track Earth's most abundant, and arguably most important, heat-trapping gas: water.
"There's no question CO2 is driving changes in our planet's climate, but a lot of the changes we are seeing are due to changes in the water cycle, and to the amount of water vapor in the air," said David Noone, a climate scientist with the University of Colorado at Boulder's Cooperative Institute for Research in Environmental Sciences. Noone, along with Assistant Professor Joe Galewsky of the University of New Mexico, are leading the water vapor-tracking project throughout the month of October.
In its vapor phase, H2O, like carbon dioxide, traps and radiates heat back towards our planet. As global temperatures rise, atmospheric humidity also increases, boosting the greenhouse effect.
Even though water vapor lasts just a short time in the atmosphere (a few days), its amplifying effects could more than double the warming caused by CO2 on a global scale, stated the Intergovernmental Panel on Climate Change in its fourth assessment report last year.
But humidity isn't expected to increase by the same amount everywhere, said Noone. Climate simulations suggest the U.S. Southwest, for example, will receive less precipitation in the future because of changes in the water cycle. This in turn has implications for liquid water resources and for global temperatures.
By helping discover what controls subtropical humidity, the group's experiments at Mauna Loa will provide important clues for understanding how changes in the water cycle influence changes in atmospheric circulation and global temperatures.
Noone suspects that if extensive drying occurs over the subtropics, roughly between 20 and 35 degrees North and South, the resulting drop in atmospheric humidity would allow proportionally more radiation to escape into space, moderating how much the planet eventually warms.
Which processes in the climate system determine the degree and extent of subtropical drying is still a largely unanswered question and the motivation behind the water-tracking experiments in Hawaii.
Noone believes three mechanisms are at play. Convection, the process that drives thunderstorm build up, draws moisture into the upper part of the lower atmosphere. In contrast, two types of large-scale atmospheric motion may be responsible for drying out the subtropics: the sinking of dry air from high up in the atmosphere and the North-South exchange of moist, tropical air and dry, polar air.
"Which of these mechanisms dominates, and whether that relationship will shift with warming, has large implications for how climate change will unfold," said Noone.
From atop Mauna Loa this October, Noone, along with CU-Boulder graduate student Derek Brown and a team from the University of New Mexico lead by Joe Galewsky, will work to identify the origins of both moist and dry air masses converging on Hawaii by measuring the chemical "tags" created by water vapor's isotopes. Isotopes of a molecule are composed of the same atoms (in water's case hydrogen and oxygen) but have different numbers of neutrons, and this affects their mass. The lighter the water, the longer the distance it has traveled since it first evaporated into the air.
The research team will use six different measurement techniques to track water vapor in real-time, relying on both field-based air samples and satellite-based remote-sensing observations. They'll be in the field October 8 - November 6.
Letter From The Field: A Chance To Go Global
Mauna Loa’s continuous CO2 measurement, spearheaded by Keeling in the 1950s, were eventually incorporated into a larger, global CO2-monitoring effort under the National Oceanic and Atmospheric Administration (NOAA) in the 1970s, with stations at Barrow, Alaska, American Samosa, and South Pole. Climate scientist David Noone hopes someday there will be a similar, global network for water vapor-tracking.
“We now have a lot more confidence in our ability to make real-time measurements of water vapor,” says Noone at the end of the month-long campaign. Back in his office at the University of Colorado at Boulder, he’s already making plots of the isotopic signatures of Mauna Loa’s moisture, using data from the trio of continuous laser-based instruments, as well as from the trap and flask samples.
“The new instruments will provide a huge amount of data, which is a great improvement over earlier sampling methods,” he says. The wealth of measurements should provide the scientists with a more accurate picture of the processes controlling Mauna Loa’s humidity.
Meanwhile, the laser-based analyzers are packed and waiting to ship back to their respective companies, where they’ll be checked once again for accuracy in the lab. The University of New Mexico is busy processing the field samples, and NASA-JPL is readying the satellite data. For now, the field research is over, but for Noone and his colleagues, the science of understanding the global water cycle has just begun.
Letter From The Field: Meet The Grad Crew
Mel – “I built this instrument and next thing I know I’m taking it to Hawaii,” says Mel while unpacking his vapor-measuring cryogenic traps. The statement is deceptively casual. In truth, Mel is a virtual MacGyver. The cryo traps, which the team is using to make validation measurements of water vapor, are the result of six months of his tinkering in the lab. The air pumps are jimmy-rigged from fish tank filters, and even the glass traps are hand blown, a skill Mel picked up from his UNM advisor Zach Sharp. Back in New Mexico, Mel collects water vapor samples for his Ph.D. research from 10,000 feet above the ground, while flying in a parachute-suspended ultralight. No big surprise that he built that too!
Derek – When the fog sweeps over Mauna Loa Observatory, Derek, a Ph.D. candidate at CU-Boulder, feels right at home. He spent a year tracking the wind, temperature, and other meteorological conditions from atop Mt. Washington, NH, known for some of the worst weather in the world. In fact, Mt. Washington still holds the record for the highest wind gust ever recorded at a surface station: 231 miles per hour! While in Hawaii, Derek is tag-teaming on the early morning cryogenic trap experiments, a bit of a change from his normal routine, which involves using climate models to understand the different convective processes that bring moisture to monsoonal regions.
John – On setup day, John wanders from one instrument group to the next, from the eclectic cryo traps to the autonomous instruments that Los Gatos and Picarro are installing. “Everyone will leave and I’ll be here, so I need to how things will work,” he says pragmatically. Not to worry; a mere couple days later, by the time the second field tests are underway, this UNM Ph.D. candidate is teaching everyone else the ropes. (See the vapor sampling video – that’s John taking notes in his yellow Rite in the Rain book!) But his comment speaks to the important role that graduate students play in research. By the time the third week roles around, the lead scientists will have left the island, leaving the project in the hands of their protégés. No sweat. This crew has it under control.
Letter From The Field: The 2:00 a.m. Shift
Every few days, TES – an emission-detecting instrument aboard NASA’s Aura satellite – peers down on Hawaii’s Big Island. By pinpointing the infrared radiation emitted by chemically unique molecules in the air, TES measures concentrations of important greenhouse gases like tropospheric ozone, methane, and, of course, both “heavy” and “light” water vapor.
To check how accurate TES’ measurements of water isotopes are, climate scientist David Noone, TES-expert John Worden of NASA-JPL, and their research colleagues are taking ground-based samples of water vapor during the satellite’s infrequent overhead flights.
And that means field sampling at 2:00 a.m.
The road to Mauna Loa Observatory winds through a red-purple sea of volcanic shards, climbing from the Saddle Road at 6,000 feet to 11,150 feet above sea level. A stomach-dropping drive during the day, the rolling, twisting one-lane road is even hairier at night.
Preparing for a 12-hour shift that will last until 5:00 a.m., the scientists and graduate students pack mugfuls of coffee. They also load up on winter jackets and outdoor gear. While at the seashore, Big Island daytime temperatures reach a steamy 80F, temperatures on Mauna Loa hover just above freezing during October nights.
The plan for tonight is to take simultaneous measurements of water vapor using two, distinct, sampling methods: flasks and cryogenic traps.
The flask, a vacuum-sealed, 2-liter, glass bauble, draws an instantaneous breath of air. With the quick turn of a valve, the job is done.
The cryogenic trap, on the other hand, is a two-hour affair, harkening back to college chemistry experiments. Built entirely of parts off eBay and home-blown glass pipettes, the traps take in atmospheric samples, freezing and condensing out the water molecules before pumping through the remaining dry air.
From 1:00 until 3:00 a.m., the glass pipettes bathe in a smoking canister of dry ice and alcohol, supercooling the air inside and crystallizing the atmospheric water vapor. (In Mauna Loa’s dry environment, two hours is a bare minimum sample-period for trapping enough water vapor to analyze back at the University of New Mexico’s isotope lab.) Every half-hour, between cups of coffee, the researchers venture out into the chilly, moon-drenched night to add more dry ice to the canister.
When the samples are finished, the intake tubes are cut and the traps stoppered with cork. They’ll be packaged and shipped back to New Mexico with the glass flasks in a few days.
Once analyzed, the flask and trap samples will provide NASA with a ground-check for what TES, orbiting 438 miles above Earth, actually “sees,” helping the researchers create a global picture of water in the air.
Letter From The Field: Set-Up Day
Within the first ten minutes of arriving at Mauna Loa Observatory, eighteen realize they're probably sixteen too many to assemble two instruments in a room no larger than a walk-in closet.("Eighteen of us working to set up an instrument we're trying to prove can run autonomously," CU climate scientists David Noone jokes.) So they kill time as any other research team would: hedging bets on the amount of "heavy" water vapor the first instrument up and running will detect.
Noone's team, a partnership of scientists and students from federal labs, universities, and industry, are setting up the first real-time measurements of water vapor's "light" and "heavy" isotopes. (Isotopes of the same molecule have different numbers of neutrons, which contribute to their total weight.) Their end goal is to understand how the water cycle may shift with climate change and influence the degree of global warming. To do that, they need to understand how moist and dry air masses traverse the globe, bringing precipitation and heat to some regions while drying and cooling others.
Once up and running, the instruments will provide a number between zero and -1000 that represents the ratio of "heavy" water isotopes in the air to "heavy" water isotopes found, on average, in the ocean. The more negative an air masses' air-to-ocean, heavy-to-light water isotope ratio, the longer and more complicated its journey. The scientists and students will use this number to identify where the various air masses that pass over Mauna Loa's massive dome originate.
"There's no other place you could be taking these measurements," says Joe Galewsky, one of the lead investigators from the University of New Mexico. He explains that air at Mauna Loa, high up in the middle of the Pacific, has probably traveled long, vertical loops from either the moist equatorial region or the cold and dry Arctic.
"An instrument here in the middle of the Pacific can tell us information about the whole Northern Hemisphere," he says.
So as far as bets go, most of the research team are leaning towards "well-traveled and dry": offering guesses between -200 and -350 for the ratio of heavy water vapor in Mauna Loa's air. Eventually they'll be able to measure the ratio in real-time. But for today, they'll just have to wait until the instruments are set up.